Firstly do you have a drivers and utilities disc may be called motherboard disc....if so run this and install the audio driver.
Goto Device Manager..
Windows key+Pause/Hardware/Device Manager now look for any yellow exclaimation marks,you may have an error code (such as code 28)
If no joy run Belarc click here and post back the motherboard details,you may need to run Everesthome to get better clarification of the motherboard click here
Also worth a try is running the codec pack, you may have missing or broken audio codecs click here